Created attachment 630617[details][diff][review]
Patch
Currently moving the splitter between the list of participants and the list of previous conversations all the way to the bottom makes a vertical scrollbar appear, and moving it all the way to the top moves the "Participants" header into the toolbar displaying the conversation title and topic.
This can easily be fixed by adding another vbox to limit how much the splitter can move the lists to the top, and adding overflow:hidden to it to avoid the scrollbar.